The invention relates to a screw fluid machine such as screw compressors, screw vacuum pumps and the like, and particularly to a screw fluid machine suitable for reducing noises.
In a twin screw compressor among screw fluid machines, it has become a significant problem to prevent noises generated by a phenomenon called "tooth separating vibration". With such twin screw fluid machines, a male rotor is made on a drive side, and lobe faces of a female rotor and the male rotor are made to come into contact with each other to drive the female rotor. Apart from this, non-contact type screw fluid machines have been often used, in which timing gears are provided on axial ends of each of the male rotor and the female rotor to mesh with each other to transmit torque. In addition, the lobe faces of the timing gears, which transmit torque, also continue to contact with each other.
Depending upon lobe profiles of the rotors and a condition of pressures acting on the lobe faces of the rotors, circumstances may happen, in which a gas torque acting on the female rotor temporarily becomes negative (here, a gas torque acting in a direction for promoting rotation is made negative), whereby there is generated a state, in which two lobe faces in torque transmitting relationship will move in relatively opposite directions. When the transmission torque again becomes positive, the lobe faces having been temporarily spaced apart from each other come to collide with each other. This phenomenon is called as tooth separation, and large vibrations and noises resulted from such repetitive tooth separation and collision are called as "tooth separating vibration".
Since large vibrations and great noises are caused by tooth separation, there have been suggested some methods of preventing the tooth separation. For example, Japanese Patent Unexamined Publication No. 5-195972 defines some conditions on lobe profiles, a rotation transmission error between rotors, and between moments of inertia on the respective rotors and rotor lobe faces, in order to prevent a transmission torque from the male rotor to the female rotor from becoming negative. Japanese Patent Unexamined Publication No. 2-252991 describes another example for preventing the tooth separation. In this example, configuration of lobe faces of a screw rotor is defined so that a transmission torque from a male rotor to a female rotor always becomes negative.
However, Japanese Patent Unexamined Publication No. 5-195972 mentioned above defines the conditions for prevention of tooth separation but does not disclose any concrete rotor configuration for satisfying the conditions, and so it is unclear what rotors can realize tooth separation. Also, Japanese Patent Unexamined Publication No. 2-252991 takes no account of gas pressure conditions such as suction pressure, discharge pressure, the kind of a gas being compressed and the like, and so satisfactory results can not be always obtained in a general use where the gas pressure conditions are changed.